It appears that in future battery systems will be used increasingly both in stationary applications and in vehicles such as hybrid and electric vehicles. In order to be able to meet the requirements placed on voltage and available power as set for a respective application, a high number of battery cells are connected in series. Since the current provided by such a battery needs to flow through all of the battery cells and a battery cell can only conduct a limited current, often battery cells are additionally connected in parallel in order to increase the maximum current. This can take place either by virtue of the provision of a plurality of cell coils within a battery cell housing or by externally interconnecting battery cells. However, it is problematic here that, owing to imprecisely identical cell capacitances and voltages, compensation currents can arise between the battery cells connected in parallel.
The basic circuit diagram of a conventional electrical drive unit, such as is used, for example, in electric and hybrid vehicles or else in stationary applications such as in the case of the rotor blade adjustment of wind turbines, is illustrated in FIG. 1. A battery 10 is connected to a DC voltage intermediate circuit, which is buffered by an intermediate circuit capacitor 11. A pulse-controlled inverter 12 is connected to the DC voltage intermediate circuit and provides, via in each case two switchable semiconductor valves and two diodes, sinusoidal currents which are phase-shifted with respect to one another at three taps 14-1, 14-2, 14-3 for the operation of an electric drive motor 13. The capacitance of the intermediate circuit capacitor 11 needs to be sufficiently high for the voltage in the DC voltage intermediate circuit to be stabilized for a period in which one of the switchable semiconductor valves is on. In a practical application such as an electric vehicle, a high capacitance in the mF range results.
If, in the case of the arrangement illustrated in FIG. 1, an asynchronous machine is used as the electric drive motor 13, it is disadvantageous that the achievable power is limited by the eddy current losses in the rotor of the asynchronous machine at high speeds. These losses are caused by the severe harmonics in the current which are caused by the high potential differences of the pulse-controlled inverter 12 and the resultant high values for the change in current over time. In order to reduce these losses, it is nowadays conventional to connect a filter 15 between the pulse-controlled inverter 12 and the asynchronous machine 13, as is illustrated in FIG. 2. By means of this filter 15, the harmonics in the current are reduced, with the result that the losses are reduced and operation at a high speed is expediently possible for the first time. However, the filter 15 increases the complexity of a required controlled system considerably, takes up installation space and also represents a considerable cost factor.